The presbyopia is generally corrected by using single-focal lenses, double-focal lenses, progressive multi-focal lenses, or the like. With single-focal lenses, however, replacement or putting-off of eyeglasses for far object is troublesome, or with progressive multi-focal lenses the visual field is narrow in each of far, middle and near ranges because of their structure, inconveniently causing a problem of shaking or distortion.
In order to solve such problems, there have heretofore been studied eyeglasses having a variable focus mechanism. For example, a focal-length variable lens disclosed in Japanese Patent Publication No. 58-50339 employs the liquid crystal. Also, an eyesight correcting apparatus disclosed in Japanese Laid-open Patent Application No. 63-254415 is so arranged that a plurality of optical lenses are laid one over another and slid relative to each other in the direction of laid surfaces to change the refracting power. However, the former focal-length variable lens using the liquid crystal has a lot of problems, such as a slow reaction speed, a variation in index of refraction of liquid crystal with temperature change, a narrow variable range of focal length, an aberration intrinsic to the liquid crystal, which plastics or glasses never have, and so on.
Then, attention is focused on the latter variable focus type eyeglasses, in which two progressive multi-focal lenses are arranged along the optical axis such that a change in refracting power of one lens is opposite to that of the other, as shown in FIG. 1, and in which the lenses are relatively slid in the direction of principal meridian to change the focal length thereof. In the specification the variable focus type eyesight correcting apparatus, in which a plurality of optical lenses are laid one over another and slid relative to each other in the direction of laid surfaces to change the refracting power, will be generally referred to as "variable focus type eyeglasses with slide mechanism."
However, if conventional progressive multi-focal lenses are employed for "variable focus type eyeglasses with slide mechanism", astigmatisms which the progressive multi-focal lenses respectively have cannot completely cancel each other, and thus it is hardly possible to secure as wide visual field as a single-focal lens has. FIG. 2 and FIG. 3 are drawings to show astigmatisms of two progressive multi-focal lenses, which constitute a lens set for one eye in "variable focus type eyeglasses with slide mechanism." FIG. 2 shows a lens on the eye ball side and FIG. 3 the other lens on the object side. When the two lenses are arranged such that O.sub.1 and O.sub.2, P.sub.1 and P.sub.2, and, Q.sub.1 and Q.sub.2 are respectively superimposed one over the other, the superimposed region of two progressive multi-focal lenses have the total astigmatism as shown in FIGS. 4A and 4B, which shows that the astigmatisms of the two lenses cannot cancel each other so as to narrow the visual field. It is known that a man cannot have a distinct vision in the range where the astigmatism is over 1 diopter. If the progressive multi-focal lens on the object side is slid from the state of FIGS. 4B in parallel with the principal meridian until O.sub.1 in FIG. 2 and P.sub.2 in FIG. 3 become laid one over the other, to change the refracting power of overlapping region between the two progressive multi-focal lenses, the range where the astigmatism is within 1 diopter becomes further narrower as shown in FIG. 5A.
The "variable focus type eyeglasses with slide mechanism" using the conventional progressive multi-focal lenses do not have only the problem of astigmatism but also a problem in power distribution. Namely, FIG. 6 shows a power distribution in a progressive multi-focal lens on the eye ball side, and FIG. 7 shows a power distribution in a progressive multi-focal lens on the object side. When the two lenses are superimposed under the same condition as in FIG. 4A, a distribution of total refracting power is not constant in the overlapping region between the two progressive multi-focal lenses, as shown in FIG. 8A. When a user wears such glasses, the user will feel unpleasant with shake or distortion of image. In addition, a part of region could not suit the prescription of the user, resulting in failure of assuring a good visual field.
Also, in order to make the "variable focus type eyeglasses with slide mechanism" smaller, a clearance between the two superposed optical lenses should better be made as short as possible, though a refractive surface on the eye ball side in the optical lens disposed on the object side could contact with a refractive surface on the object side in the optical lens disposed on the eye ball side. After the lenses are repeatedly slid in this state, contacting surfaces or points will have flaws very easily, which was another problem.
It is, therefore, an object of the present invention to provide an eyeglass lens for "variable focus type eyeglasses with slide mechanism", which can solve the problems as described above.